Nmap is sweet. Comes both commandline (fast) and GUI (great for showing non techies what is going on). Lets you map out hops between Lans visually, lets you scan what is ON your LAN... I have used this to find rogue machines, and lets you see what services are running on what ports on machines on the Lan. Includes OS's and service packs etc. A very powerful tool, and fun to play with.

NMAP is a great tool. I can't imagine not to use it. The best book, to me, to learn more about nmap is Fyodor's http://nmap.org/book/toc.html. A very professional book that will give you insights to topics like host discovery.